Electric power steering provides a steering assist to a motor vehicle driver as the driver turns the steering wheel in either direction of rotation. The electric motor of the electric power steering (EPS) system which serves to assist the steering by the driver can be connected to the rack of the steering system (an REPS system) or be connected to the steering column (a CEPS system), which are exemplified at FIGS. 1A and 1B.
FIG. 1A depicts an example of a CEPS system. A motor vehicle 40 is provided with an electric power steering system 24. The electric power steering system 24 may comprise a conventional rack and pinion steering mechanism 36, which includes a toothed rack (not shown) and a column pinion gear (not shown) under gear housing 52. As the steering wheel 26 is turned, an upper steering shaft 29 turns a lower shaft 51 through a rotary joint 34; and the lower steering shaft 51 turns the column pinion gear. Rotation of the column pinion gear moves the rack, which moves tie rods 38 (only one shown), which move steering knuckles 39 (only one shown) to turn tires 42 (only one shown).
The electric power assist is provided through a controller 16 and a power assist actuator comprising an electric motor drive 46. The controller 16 receives electric power from a vehicle electric power source 10 through a line 12, a signal representative of the vehicle velocity on line 14 and column pinion gear angle from a column rotational position sensor 32 on line 20. As the steering wheel 26 is turned, a torque sensor 28 senses the torque applied to steering wheel 26 by the vehicle operator and provides an operator torque signal to controller 16 on line 18. In response to the vehicle velocity, operator torque, and in some cases, column pinion gear angle signals received, the controller 16 derives desired electric motor currents and provides such currents through a bus 22 to the electric motor drive 46, which supplies torque assist to steering shaft 29 through worm gear 47 and motor pinion gear 48. Details hereof are described in U.S. Pat. No. 5,982,067, issued to Sebastian et al on Nov. 9, 1999, the entire disclosure of which is hereby herein incorporated by reference. An example of an embodiment of the controller 16 is described in U.S. Pat. No. 5,668,722, issued to Kaufmann et al on Sep. 16, 1997, the entire disclosure of which is hereby incorporated herein by reference.
FIG. 1B depicts an example of an REPS system. The electric power steering system 60 comprises a conventional rack and pinion steering mechanism 62, which includes a toothed rack 64 which is connected to the tie rods (not shown) for directing the turning of the tires (not shown). The steering column has a lower assembly 66 having a column pinion gear 68 which is meshed with the teeth 70 of the toothed rack 64 so that turning of the steering column applies a torque at the toothed rack that results in the toothed rack translating left or right, depending on the direction of the turning of the steering column. The electric motor drive 72 of the electric power steering system is gearingly connected to the toothed rack by a motor pinion gear 74, wherein the motor pinion gear may be mechanically connected, for example, by a belt or gear interface via, for example, a ballscrew mechanism 76. The electrical operation is as generally described with respect to FIG. 1A, as it is adapted to the configuration of FIG. 1B.
Additional sensors for both CEPS and REPS are available and oftentimes implemented to yield motor rotor position, enabling differentiation of these signals with respect to time, ultimately providing estimates of rotor velocity and acceleration. The use of these differentiated signals to provide advantageous electrical control characteristics is described in subsequent paragraphs and will become obvious to the reader. In the case of some brushless motor mechanizations, furthermore, the rotor position is also used for distributing power magnetically to a rotor whose position must be known relative to a stationary member, e.g.: stationary windings interacting with a rotatable permanent magnet rotor. Multiple motor configurations are possible for achieving desired mechanical torque at the rotor, such as brushed motors, inductive motors, and synchronous motors as examples. These mechanizations and the practices associated with production of mechanical torques between stationary and rotatable members of motor elements are well known to those skilled in the art of electric motors for not only steering systems but, in general, the creation of mechanical torques through actions of electric motors in appliances, fans, flywheels and other industrial machines using rotatable prime movers.
Nonuniform conditions of the rotating tire, wheel, brake rotor and bearing hubs of a motor vehicle may cause periodic vibrations, in isolation of, or in addition to road induced vibrations on even extremely smooth road surfaces. These vibrations, furthermore, can exhibit a recurrent, periodic torsional vibration at the steering wheel, commonly referred to as “shake”, wherein this shake is more pronounced with increasing speed and is most noticeable at speeds greater than, approximately, 50 miles per hour (mph). These nonuniform, periodic conditions cause the rack of the steering system to vibrate with a periodicity related to the periodicity of the shake and is, generally, most notable between about 10 and 20 Hz. The shake may be felt by the driver at the steering wheel as a periodic rotational vibration, known as “smooth road shake” (SRS), generally, most noticeable between about 10 to 20 Hz at speeds, generally, between, approximately, 50 to 100 mph. At 50 mph, smooth road shake occurs at, approximately, 10 Hz, the frequency of which is an approximate linear function of speed such that at 100 mph smooth road shake occurs at, approximately, 20 Hz.
Accordingly, what is needed in the art is some methodology which provides for the attenuation of smooth road shake, particularly a dynamic attenuation responsive to differing speeds of the motor vehicle.